Well I've got a few concepts along these lines that I'm interested in developing further...direct MILF or TG concepts, etc. Got a TG piece coming from AutumnNatural that explores a few looks a little more too - whether in period clothing/etc...
It's kind of interesting to see exactly where the age concepts break out - between something like a 30ish MILF or something older like the portrayals in bamtfartist's work. There was also a story I found on fictionmania a few weeks ago (can't remember the title now) but it involved a younger guy going for a sex change operation with additional physical changes to make him resemble an older woman as well....like his mother's age or even older. A little different.
There's also something like the story "Payback's a Cougar" -
http://www.mcstories.com/PaybackIsACougar/index.html
I've been considering getting that one illustrated...or a similar transformation progression sequence.